MBS is a point-to-multipoint service transmitting data from a service source to a plurality of receivers through a common wireless channel. The IEEE802.16e-2005 standard has defined the MBS in a mobile WiMAX (Worldwide Interoperability for Microwave Access) network, which can be implemented as a cost-effective way to provide mobile TV or Video on Demand (VOD) services.
According to the definition in the IEEE802.16e standard, a multi-base-station MBS can be supported by constructing separate regions in the downlink frame.
FIG. 1 is an exemplary diagram showing the network architecture of multi-base-station MBS in a wireless network in the prior art. As shown in FIG. 1, a system for providing a MBS comprises a content pool, an MBS controller, a plurality of base stations (three, BS1, BS2 and BS3 in this example) and a plurality of mobile stations. The MBS controller provides the plurality of base stations with MBS data from the content pool and performs authentication of mobile stations and distribution of encryption key for the MBS. The plurality of base stations provide the plurality of mobile stations with the MBS data transferred from a backbone network through common air interface. The plurality of mobile stations receive the MBS data transmitted from the serving base station they connect.
For the access mode of multi-base-station MBS, a single frequency network (SFN) with flexible duration of MBS regions is employed to permit the scalable assignment of radio resources to MBS traffic. Each base station is capable of providing MBSs belonging to the same MBS zone where a set of base stations with the same connection index (CID) and the same security association (SA) are used for transmitting content of certain service flows. The multi-base-station MBS in SFN does not require a mobile station to register with a new base station during a roaming process, which can reduce a handover delay and a service broken.
To enable a mobile station to receive MBSs from a plurality of base stations, MBSs need to be strictly synchronized across these base stations. However, the synchronization of MBS among multiple base stations is hard to achieve because of the following four constrains: the identical Packet Data Unit (PDU) format and CID assignment; the identical Orthogonal Frequency Division Multiple Access (OFDMA) frame; the identical OFDMA data region; and the identical channel coding scheme.
Except for the overhead for the content synchronization such as functions of classification, fragmentation and centralized scheduling, there are the following drawbacks for deployment of MBS in SFN.
(1) Waste of MBS Bandwidth
As long as a MBS zone is created, a multicast service that starts up inside one base station should be extended to all base stations belonging to the same MBS zone. Obviously, for some non-popular programs with limited subscribers located inside a small amount of base stations, the bandwidth will be wasted in the cell where there is no subscriber for this program.
(2) No Flexibility for Link Adaption Among Multiple Base Stations
How to fulfil a link adaption for MBS in a single-base-station access is a challenging task because of the great amount of overhead for feedback channel maintenance. It is nearly impossible to have the link adaption among multiple base stations since there is a remarkable variation of subscriber distribution. Accordingly, the conventional method skips the link adaption and chooses the most robust (fixed) modulation and coding scheme for MBS. The disadvantage is that the system utility will be reduced a lot due to the poor spectrum efficiency of low level modulation and coding scheme.
(3) Affection on Unicast Service
Normally, the MBS will cover partial downlink sub-frame. And in a WiMAX system, a base station needs to allocate uplink transmission opportunity for a mobile station when there is traffic originating from the mobile station. It is difficult for the base station to assign a symbol and a subchannel for a unicast service in the SFN because of the unpredictable interference from neighboring base stations operating in the same frequency band. As shown in the FIG. 1, there are overlapping areas among neighboring base stations BS1, BS2 and BS3 in the backbone network. For those mobile stations located in these overlapping areas, the transmission of unicast service to/from its serving base station is inevitably interfered with the unitcast traffic in the area of the neighboring base stations. Thus the overall system utility will also be affected.
A multiple frequency network (MFN) is another approach for multi-base-station access. Although the MFN can overcome the above described disadvantages of the SFN, a distinct challenge is the uncertain handover processing time during the course of a mobile station switching from one base station to another. In such case the mobile station needs to terminate the existing connection with the serving base station, go through the network entry procedure and then re-establish the service connection with a new base station. Because the content synchronization is not a mandatory requirement in MFN environment, even the content source is from the same MBS controller via the backbone network, the delay from MBS controller to each base station might be different. Thus the following two problems should be considered for the MBS handover process in WiMAX network:
(1) Minimizing the switching time for handover
(2) Minimizing the service interruption.
For the problem 1, there are some mechanisms already defined in the IEEE802.16e standard to reduce the switching time. For example, according to one mechanism, if a mobile station have previously received neighboring advertisement message from its serving base station, it can acquire the information including the ID, the physical frequency and the downlink/uplink channel description of the candidate target base station. Thus the synchronization to the target base station can be achieved in a quick way. Furthermore, the network re-entry proceeds may also be shortened if the target base station can possess information of the mobile station from the serving base station over the backbone network, by which the authentication and registration phases can be skipped.
For the problem 2, there is no method mentioned in the IEEE802.16e standard for data and service integrity. A mobile station must terminate the connection with the serving base station before the handover processing. No function model is defined for the base station to support seamless MBS handover in MFN mode.
Full information of the MBS in a mobile WiMAX network cab be referred by the IEEE802.16e standard.
In summary, the current definition in the IEEE802.16e standard uses the SFN for multicast and broadcast service among multi-base station access. However, such method will not only bring great overhead for content and frame synchronization among base stations grouped in the same MBS zone, but also cause bandwidth waste and efficiency descending for unicast services. By contrast, the MFN as described above can reduce overhead for content synchronization and minimize the interference for unicast services in the cell boundary and overlapping area. But the major problem of the MFN is the possible interruption of service during the handover processing when switching from one base station to another.
Therefore, the problem of an efficient and seamless MBS handover for multi-base-station access in MFN needs to be addressed.